Aldehydes are compounds containing carbonyl group. When two hydrogens, or one hydrogen and one alkyl group are attached to the carbonyl the compound is an aldehyde. Aldehydes are syntheseised by oxidation of aliphatic alcohols of the general formula ROH, R being any alkyl group, whereby the alcohol is reacted with oxidizing agents like chromium (VI) oxide, potassium di chromate at a temperature not exceeding 40° C. The process results in the aldehydes of wide ranging molecular weights that may range between 10000–20000 D. This conventional process however requires that the reaction temperature have to be controlled to prevent the oxidation of generated aldehydes into carboxylic acids.
As reported by C. A. Buehler and D. E. Pearson (Survey of organic syntheses, Wiley-Interscience Publication, Chapter 10, 549,1970) aldehyde has been prepared from primary alcohol by using catalyst containing cobalt 5% and chromium 2%.
Also reported by C. A. Buehler and D. E. Pearson (Survey of organic syntheses, Wiley-Interscience Publication, Chapter 10, 549,1970) acetaldehyde has been prepared by three-step process, whereby glycerol is first reacted with acetone for projecting two of the hydroxyl groups followed by the oxidation of the resulting acetone glycerol over silver catalyst for 24 hours and subsequent hydrolysis of the product.
Thus aldehyde has conventionally been synthesized normally from non-proteinous source. It has been used for different industrial applications. Different aldehydes have been in use in the leather processing industry as tanning, retanning and even filling agent. The cost involved in the preparation of these chemicals from the conventional costly organic compounds poses a hindrance to the growth of the leather chemical industry involving aldehydes. Another limitation associated with these aldehydes is that some of the compounds, required for the preparation, have potential risk of being hazardous in respect of health and/or environment. Several attempts are therefore being made, in view of the growing concern for eco-sensitive scenario in the present age, to explore new sources for preparing aldehydes for application in different industries including that of leather processing. Several successful attempts have been made by different research groups to break down proteinous materials into the respective hydrolyzates.
Ramamurthy et al (Journal of society of leather technologists and chemists, 73,168,1985) provided a process for the preparation of keratin hydrolyzate, whereby a keratinous source is hydrolysed by 10% of alkali or acid or 0.25–0.5% of enzyme. The resulting product finds application in leather processing as syntan, filler and also as a chrome exhaust aid. The major limitation associated in this hydrolyzate is that the proteinous material itself is a limitation to its shelf life, which cannot go beyond 6 months.
Reference may be made to Kanagaraj et al (journal of American Leather Chemists Association, 97, 207–214, 2002) and our co-pending Indian Patent application No.210/DEL/2001, wherein a process has been provided for the preparation of protein based acrylic syntan, which has the wide application in leather processing. The syntan is prepared by hydrolyzing the protein source with acid or alkali followed by adjusting the pH to 6–7. The hydrolyzed protein source thus obtained is coplymerised with acrylates to obtain the acrylic syntan. The advantages of the acrylic syntan thus obtained are that it can be used as a filling agent, grain tightening agent and can withstand the temperature of 150° C. during glazing. The acrylic syntan increases the exhaustion of dye to the level of 80–90% in the resulting leather. But the limitation associated with the acrylic syntan is that it cannot be stored for more than 1 year. Moreover, the higher molecular weight of the syntan in the range of 15000–20000 D poses a problem in penetration. Another limitation associated with this is that it cannot be used as tanning agent because of the absence of any crosslinking agent and the tendency of the proteinous substance to decay. Thus both keratin hydrolyzate and protein based acrylic syntan cannot be used as self tanning agent.
Another copending Indian patent application No. 1063/DEL/2001 has provided a process for the preparation of novel proteinoid for industrial application which has the wide application in leather industry. The material is prepared by hydrolyzing a protein source with alkali and adjusting the pH of 6–7 and then subjected to oxidation by using organo-oxidizing agent to form reactive proteinoid. Here the molecular weight of the product is found to be as high as 7000–10000 D, hindering the penetration of the syntan into collagen fibres.
All these above attempts made on the proteinous source have thus been associated with a major limitation of partial break down of protein, resulting in the formation of significantly larger molecules, which are not suitable in leather processing. As reported by Herfield (Vegetable tannage, chapter 3, 3, 1986) the typical molecular weight of material that is suitable for tanning the pelt is in the range of 3000–5000 D.